This invention relates to inverter circuits for starting and ballasting gas discharge lamps. More particularly, this invention relates to inverter circuits for producing an alternating current output which is regulated to a constant power level as a load impedance changes.
Inverter circuits are commonly used to produce alternating current power from a direct current source and to effect changes in the voltage level of an alternating or direct current source. It has been common practice to incorporate circuits in inverters which act to regulate and maintain the output current and/or the output voltage of the inverter at a constant value.
It is, in some instances, desirable to utilize an inverter circuit which will supply a constant power level to a load despite changes in the load impedance. Inverters with constant power output are useful, for example, for driving gas discharge lamps in which impedance characteristics vary as a function of the lamp operating conditions. A constant power output is also desirable for charging of energy storage capacitors of the type commonly utilized in photoflash and and pulse modulator equipment and for operating variable speed motors at constant power levels.
U.S. Pat. No. 3,781,638 by Thomas E. Anderson and John P. Walden describes a class of alternating current inverter circuits wherein commutation of an output voltage is initiated as load current reaches a predetermined, reference level. Inverters of this class will, therefore, function to regulate and maintain the power input to a load of constant impedance despite changes in the inverter input voltage. The above-mentioned patent disclosure is incorporated herein by reference.
The operation of gas discharge lamps with frequency controlled inverters and resonant matching networks is more particularly described in concurrently filed patent applications Ser. No. 662,529 by William P. Kornrumpf et al. and Ser. No. 662,523 Thomas A. Anderson, which are assigned to the assignee of this invention and incorporated herein by reference. In accordance with those disclosures, a gas discharge lamp is connected as a damping element across a capacitor of an otherwise high Q series resonant circuit. Prior to ignition, the lamp presents a very high impedance and the Q of the resonant circuit remains high. The circuit is automatically driven at its resonant frequency during this period. Voltage buildup in the high Q circuit provides high voltage necessary for lamp starting. After ignition, the lamp impedance decreases greatly, loading the resonant circuit and lowering its Q. At such times the inductor or a current control circuit act to limit current flow through the negative lamp impedance. Such circuits may not, however, maintain circuit resonance to permit reignition of a partially ionized lamp having an intermediate impedance value.